Apparatus for operating gaseous discharge lamps on direct current from a source of alternating current

ABSTRACT

A system providing flickerless operation of a gaseous discharge lamp utilizes a DC voltage applied across the lamp electrodes. Low starting voltage and power consumption are obtained by applying an AC voltage across a grounded conductor member and adjacent lamp electrodes.

United States Patent Ott 1 1 June 17, 1975 [54] APPARATUS FOR OPERATINGGASEOUS 287L412 1/1959 Lord 1 1 1 e 1 1 1 v 1 315/205 DISCHARGE LAMPS 0NDIRECT CURRENT l :32? 29 2; ig 2 rlec aum t .1 FROM A SOURCE OFALTERNATING 3,066,243 I 1/1962 Mutschler 315/176 CURRENT 3,089,9795/1963 Lovinger 315/85 75 Inventor; Hem-y Whitfield o Sarasota Fla.$222,570 12/1965 Clark et a1. SIS/DIG. 5 3,238,415 3/1966 Turner 1.315/160 [73] Assignee: John Ott Laboratories, Inc., 3.467 886 9/1969Alley 315/163 Sarasota, Fla. $476,977 11/1969 Hallay 315/183 3,514,6675/1970 Dessovlavy et a1. 3l5/D1G1 S 1221 F'ledi 19, 1974 3.721850 3/1973Daniels et a1 321/47 [21] Appl. No.:

Primary ExaminerGerald Goldberg Attorney, Agent, or Firm-Richard E.Hosley [52] US. Cl. 315/335; 315/171; 315/257; 315/262; 315/D1G. 5 15111111. C1 1105b 41/04; HOSb 41/19 [57] fBSTRACT [58] Field of Search2115/1310. 5, 175, 176, 257, A system p s fllckerless Operation of agaseous 315 335 33 2 1 2 ,5 2 5, no 205 m3 discharge lamp utilizes a DCvoltage applied across 35/133 1 0 5 the lamp electrodes. Low startingvoltage and power Consumption are obtained by applying an AC voltage 56]References Cited across a grounded conductor member and adjacent UNITEDSTATES PATENTS electrdes- 2,864 03S Davis 3. 315/335 x 9 Claims, 1Drawing Figure LAMP j'j" 1 APPARATUS FOR OPERATING GASEOUS DISCHARGELAMPS ON DIRECT CURRENT FROM A SOURCE OF ALTERNATING CURRENT BACKGROUNDOF THE INVENTION The present invention relates to apparatus for startingand for operating, on direct current, gaseous discharge lamps such asfluorescent lamps from a source of alternating current.

Fluorescent lamps are widely used for domestic and industrial lightingbecause they generate less heat and are more efficient than other commonlight sources such as incandescent lamps. They are ordinarily operatedon alternating current (AC) because AC power sources are readilyavailable and because AC systems provide certain advantages inballasting and obtaining adequate starting voltage for the lamp.However, AC operation of fluorescent lamps has several disadvantages.Because the arc in the lamp must strike at twice the frequency of the ACsupply current a flickering light is produced. Thus, when operated on 60Hz current, the lamp produces 120 flashes per second. This flickeringeffect can cause headache and eye-strain in some individuals and cancause epileptics to go into seizure. Also, because of the rapid rise inarc current in the lamp at each flash, the lamp emits a broad band ofradio frequency radiation which causes interference problems in lightingapplications where there is sensitive electronic equipment in an arealighted by fluorescent tubes.

To obviate the above difficulties of light flicker and unwantedradiation gaseous discharge lamps can be operated on direct current andthis has been done where direct current (DC) power sources areconveniently available as on some subway trains. However, the availablepower supply is ordinarily AC and this requires the use of specialrectifier, ballast and starting circuits for the lamp which haveheretofore been expensive and relatively inefficient. This has limitedthe use of DC operated fluorescent lamps to a few special cases where DCpower supplies are available or special applications such asphotoprinting lights where the extra cost of the power rectificationapparatus can be justified.

The reason prior apparatus for operating gaseous discharge lamps on DCfrom an AC source has been expensive and relatively inefficient stemsfrom the electrical operating characteristics of such lamps. It is wellknown that they have an inherent negative resistancecurrentcharacteristic so that after the lamp arc strikes, the lamp current willbecome excessive unless a suitable ballasting device is used. Ballastsused in DC systems heretofore have caused substantial power loss oftenexceeding the loss in the lamp itself. Also, such lamps normally have astarting voltage substantially higher than the operating voltage. In DCsystems the rectifiers must be able to withstand the high startingvoltage or alternatively some automatic control system must be providedto isolate the rectifiers from the high starting voltage. Also, in suchsystems the starting voltage has been obtained by using apparatus suchas a pulse transformer for superimposing AC on the DC current suppliedto the lamp electrodes. This AC will cause the lamp to flicker unlessmanual or automatic apparatus is provided to remove it from the circuitafter starting. Such apparatus adds considerably to the cost.

Accordingly, it is an object of the invention to provide improvedapparatus for operating gaseous discharge lamps on DC from a source ofAC that is less costly and more efficient than apparatus heretofore usedfor this purpose.

Another object of the invention is to provide an improved circuit andapparatus for operating gaseous discharge lamps without flicker andwithout objectionable radiation in the radio frequency range.

A still further object of the invention is to provide apparatus forstarting and operating gaseous discharge lamps that does not require aballast transformer or series reactor to obtain stable operation.

Further objects and advantages of the invention will become apparent asthe following description proceeds.

SUMMARY Briefly, in accordance with the invention the lamp starting andoperating apparatus comprises a rectifier having an input circuitenergized from a source of AC through a conventional voltage-changingtransformer. The output circuit of the rectifier is connected to thelamp electrodes to provide a DC operating current for the lamp. Toreduce the starting voltage and assist in the starting of the lamp agrounded conductor member is placed in close proximity to the lamp sothat a small capacity exists between the conductor member and theelectrodes of the lamp. Circuit connections to the transformer are suchthat an AC voltage exists between the conductor member and the lampelectrodes causing a small capacitive current to flow in the conductorbut not in the lamp arc current flowing between the electrodes so thatit does not produce lamp flicker. A small series ballast resistor in therectifier output circuit is used to stabilize the lamp current.

For a better understanding of the invention, reference should be made tothe following detailed description taken in connection with theaccompanying draw- BRIEF DESCRIPTION OF THE DRAWING The single FIGURE ofthe drawing is a circuit diagram showing apparatus in schematic form forstarting and operating a fluorescent lamp which embodies the presentinvention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Referring now to the drawing,there is shown apparatus for starting and for operating on directcurrent a fluorescent lamp 10. The lamp shown is a common hot cathoderapid start type having electrodes 11 and 12 at opposite ends of thetube envelope 13. The electrodes are a coiled filament type which arecontinuously heated during the lamp operation. The lamp is energizedfrom a suitable source of alternating current (AC) supplied to inputterminals 14 and 15.

The apparatus utilizes a conventional voltagechanging power transformer16 comprising a magnetic core 17 on which are wound in inductiverelation, a primary winding 18 and three secondary windings 19, 20 and21. The primary winding 18 is connected to the AC input terminals 14 and15 through a lamp starting switch 22.

A full wave rectifier, indicated generally by number 23, is provided forfurnishing direct current (DC) to the electrodes 11 and 12 forflickerless operation of the lamp. The rectifier, in the formillustrated, has two diodes 24 and 25 whose cathodes are coupled to thepositive side 26a of the rectifier output circuit. The anode of therectifier diode 24 is connected to one end connection 26 of thetransformer secondary winding [9 while the anode of the other rectifierdiode 25 is connected to a winding tap connection 27. An intermediatewinding tap connection 28 of the winding 19 is connected, as shown, tothe negative side 29 of the rectifier output circuit. The secondarywinding 19 of the transformer 16 may be considered as twoseries-connected sections with a first section extending betweenconnections 26 and 27 and a second section extending between connectors27 and 40. As so considered it will be noted that the rectifier 23 isconnected to the first section. The function of the second section willbecome apparent as the following description proceeds. The positive andnegative sides 26a and 29 of the rectifier output circuit are connectedto the lamp electrodes 11 and 12 through a polarity reversing switch 29.By reversing the polarity of the current supplied to the lamp electrodeswith this switch, the degradation of the lamp cathodes and the migrationof the phosphor coating on the inside of the tube can be equalized in aknown manner. The polarity reversing switch can be manually orautomatically operated.

The cathode heaters of the electrodes 11 and 12 are energized from theheater windings 20 and 21 of transformer 16. In order to eliminate allflicker from the lamp, the cathode heaters are preferably energized withdirect current. To provide such current rectifier diodes 30 and 31 areplaced in the heater circuits along with the usual filter capacitors 32and 33. Alternatively, switches may be provided to disconnect the heatercircuits after the lamp has started.

In order to remove any AC ripple from the lamp arc current flowingbetween lamp electrodes 11 and 12 a filter capacitor 34 is connectedacross the positive and negative sides 26 and 29 of the rectifier outputcircuit and this capacitor is bridged by the usual bleeder resistor 35.Also included in the rectifier output circuit is a series-connectedballast resistor 36. The voltage drop across the resistor acts tostabilize the arc current of the lamp which would otherwise tend to runaway due to negative resistance-current characteristic of fluorescentand other types of gaseous discharge lamps.

The starting voltage necessary to start a gaseous discharge lamp isnormally substantially higher than the operating voltage required tomaintain the flow of current through the lamp after the arc has beenstruck. In prior DC gaseous discharge lamp systems energized from ACsupplies, this additional starting voltage has been obtained bysuperimposing on the DC rectifier output an AC voltage obtained from apulse transformer or the like. This AC voltage must be removed after thelamp starts or it will cause a flicker in the lamp light output. Thisadds complication and cost. Also high leakage reactance transformers orballasts have been required to provide a high open circuit startingvoltage which drops after the lamp current starts to flow. Usually thereis substantial power loss in such high reactance transformers orballasts which reduces the efficiency of the system. These disadvantagesare obviated in the present system by applying both AC and DC to thelamp electrodes during both starting and running conditions as will nowbe described.

To reduce the starting voltage and aid in starting of the lamp, there isprovided a conductor member 37 which extends along the length of thelamp in closely spaced relation therewith so than small capacity existsbetween the conductor member and the lamp electrodes 11 and 12. Theconductor member is connected by a lead 38 to a ground connection 39 andalso to an end terminal 40 of the second section of the secondarywinding 19 of the transformer 16. Because the outer end connection 40 ofthe transformer secondary winding 19 is grounded, the potential of lampelectrodes 11 and 12, which are connected to the first winding sectionoperating at different potential by virtue of the series connectionbetween the winding sections, alternately swings above and below groundat the frequency of the AC supply. Thus an AC voltage is applied acrossthe conductor member 37 and the lamp electrodes which causes a smallcapacitive current to flow therebetween. Since this current does notflow in the DC are current flowing between the lamp electrodes, it doesnot cause any lamp flicker. Because of this and the fact that thecapacitive current is small the AC voltage across the conductor memberand lamp electrode can be left on during lamp operation without flickereffects or any appreciable power loss. Thus no manual or automaticswitching equipment is required to disconnect the starting voltage.

in many applications a metallic lamp fixture in which the lamp 10 ishoused may be used as the conductor member 37. Since the conductormember is at ground potential, there is no shock hazard. In otherapplications a conductor strip may be fastened along the length of thelamp envelope.

It has been found that the combination of DC voltage applied across thelamp electrodes and AC voltage applied across the conductor strip andthe lamp electrodes has the effect of substantially reducing the voltageacross the electrodes required to start the lamp. Thus with an 18 inch,15 watt rapid start fluorescent lamp of the type illustrated and withabout I volts AC applied across the conductor member and the lampelectrodes, the lamp was found to start consistently with a DC startingvoltage of only volts as compared with a normal starting voltage ofabout I 10 volts. Also the lamp was found to operate in a stable mannerand without flicker with a ballast resistor, 36 having a resistancevalue of only 10 ohms. Thus the power loss in the resistor wasnegligible.

Because of the low starting voltage a ballasting transformer or reactorand high voltage rectifiers are not required. Thus the power loss andextra cost associated with such devices is avoided. The DC in the lamparc and heater circuits eliminates lamp flicker and since the arc issteady rather than oscillating radio frequency noise problems areeliminated.

From the foregoing, it will be apparent that there has been provided asystem and apparatus for flickerless operation of a gaseous dischargelamp that is simple, efficient and can be manufactured at a low cost ascompared with previous equipment for performing the same function. Thesystem may be used with other types of metallic vapor arc dischargelamps which normally require high starting voltages and have negativeresistance-current characteristics.

While there has beenshown what is presently considered to be a preferredembodiment of the invention, it will be apparent to those skilled in theart that various changes and'modifications may be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Apparatus for starting and operating a gaseous discharge lamp so asto provide flickerless operation of the lamp, said lamp having a pair ofelectrodes across which a voltage is applied to start and operate thelamp, said apparatus comprising:

a. a conductor member placed in close proximity to the lamp so that asmall capacity exists between the conductor member and the lampelectrodes,

b. power transformer means adapted to be connected to an AC power sourcecomprising two seriesconnected secondary winding sections,

c. rectifier means having an AC input circuit connected to one of saidsecondary winding sections and an output circuit connected to the lampelectrodes for operating the lamp on a DC current flowing between thelamp electrodes, and

d. circuit means connecting said conductor member to the other of saidsecondary winding sections at a potential point such that an AC voltageis applied across the conductor member and the lamp electrodes whereby acapacitive current flows therebetween to aid starting of the lampwithout causing an AC ripple in the DC current flowing between the lampelectrodes.

2. The apparatus of claim I wherein one of the secondary windingsections of the transformer has two end connections and one intermediatetap connection with the rectifier input circuit connected to the two endconnections and the rectifier output circuit connected to theintermediate tap connection and the conductor member connected to theouter end of the other winding section.

3. The apparatus of claim 1 wherein the conductor member is grounded.

4. The apparatus of claim 1 wherein the lamp electrodes have heaterwindings associated therewith and means for energizing the heaterwindings with DC.

5. The apparatus of claim 1 wherein the DC output circuit of therectifier means includes a series ballast resistor for stabilizing thelamp.

6. The apparatus of claim 1 wherein the output cir cuit of the rectifiermeans includes a polarity reversing switch.

7. The apparatus of claim 5 including a filter capacitor and a bleederresistor connected across the rectifier output circuit.

8. The apparatus of claim 4 wherein the heater windings are energized byheater circuits connected to secondary windings on the transformer, theheater circuits including rectifiers so that DC is supplied to theheater windings.

9. The apparatus of claim 8 wherein the output circuit of the rectifierincludes a polarity reversing switch.

1. Apparatus for starting and operating a gaseous discharge lamp so asto provide flickerless operation of the lamp, said lamp having a pair ofelectrodes across which a voltage is applied to start and operate thelamp, said apparatus comprising: a. a conductor member placed in closeproximity to the lamp so that a small capacity exists between theconductor member and the lamp electrodes, b. power transformer meansadapted to be connected to an AC power source comprising twoseries-connected secondary winding sections, c. rectifier means havingan AC input circuit connected to one of said secondary winding sectionsand an output circuit connected to the lamp electrodes for operating thelamp on a DC current flowing between the lamp electrodes, and d. circuitmeans connecting said conductor member to the other of said secondarywinding sections at a potential point such that an AC voltage is appliedacross the conductor member and the lamp electrodes whereby a capacitivecurrent flows therebetween to aid starting of the lamp without causingan AC ripple in the DC current flowing between the lamp electrodes. 2.The apparatus of claim 1 wherein one of the secondary winding sectionsof the transformer has two end connections and one intermediate tapconnection with the rectifier input circuit connected to the two endconnections and the rectifier output circuit connected to theintermediate tap connection and the conductor member connected to theouter end of the other winding section.
 3. The apparatus of claim 1wherein the conductor member is grounded.
 4. The apparatus of claim 1wherein the lamp electrodes have heater windings associated therewithand means for energizing the heater windings with DC.
 5. The apparatusof claim 1 wherein the DC output circuit of the rectifier means includesa series ballast resistor for stabilizing the lamp.
 6. The apparatus ofclaim 1 wherein the output circuit of the rectifier means includes apolarity reversing switch.
 7. The apparatus of claim 5 including afilter capacitor and a bleeder resistor connected across the rectifieroutput circuit.
 8. The apparatus of claim 4 wherein the heater windingsare energized by heater circuits connected to secondary windings on thetransformer, the heater circuits including rectifiers so that DC issupplied to the heater windings.
 9. The apparatus of claim 8 wherein theoutput circuit of the rectifier includes a polarity reversing switch.